The present invention is in the field of ceramic sealing, and particularly relates to improved suspensions of fritted solder glasses useful for sealing together glass or other ceramic parts.
Solder glass suspensions have been used in the fabrication of many different kinds of assemblies comprising ceramic parts, including, for example, glass envelopes for electrical devices, glass-ceramic components for telescope mirror blanks, and ceramic substrates for electrical circuits. Such suspensions are typically provided in paste form, comprising a major proportion of fritted solder glass and a minor proportion of a pyrolyzable organic vehicle which burns away during the process of firing to effect sealing between the solder glass and the ceramic parts to be sealed.
One of the most demanding applications for solder glass suspensions is in the manufacture of cathode ray tube envelopes for television picture tubes and the like. Such envelopes are fabricated by sealing together a glass funnel member and a glass face plate or panel member. A solder glass suspension is applied by extrusion from a reservoir through an orifice in the shape of a continuous ribbon or bead to at least one of the mating surfaces provided on the panel and funnel, and these parts are joined and fired to provide an integral assembly wherein the fused solder glass provides a mechanically strong, high dielectric strength, hermetically sealed joint.
For this and similar applications, the pyrolyzable organic vehicle for the solder glass suspension typically comprises a cellulose binder and an organic solvent for the binder, e.g., a nitrocellulose binder with an amyl acetate solvent. Fritted solder glass additions of about 10-15 parts by weight of fritted glass for each part of organic vehicle are typically used to provide a suspension having the consistency of a paste, suitable for controlled dispensing onto a substrate to be sealed.
Although the use of suspensions of this type in the television industry is widespread and of long standing, several problems associated with such use remain. One problem is that, with time, such suspensions tend to separate into their respective glass and vehicle components, necessitating remixing prior to or during use.
Another problem is that the viscosity of such suspensions can change substantially over a relatively short time interval. In presently used systems for dispensing these suspensions, the dispensing rate is directly affected by the viscosity of the suspension, so that frequent adjustment of the dispensing machinery is required in order to achieve uniform and reproducible extrusion of the suspension onto a substrate. Also, the reductions in viscosity which typically occur on aging often produce sagging or running of the suspension after deposition on the substrate, resulting in a messy or mechanically defective seal.
These undesirable characteristics severely limit the useful life of presently used solder glass suspensions, typically to time periods on the order of hours or, at most, a few days. Accordingly, such suspensions are presently prepared at the point of use, in relatively small batches of a size which can be quickly used.
It is a principal object of the present invention to provide improved solder glass suspensions offering extended useful life and dispensing characteristics which are stable with time.
It is a further object of the invention to provide a method for extending the useful life of conventional solder glass suspensions from the present periods on the order of hours to periods of several months or more.
Other objects and advantages of the invention will become apparent from the following description thereof.